A taper tap for pipe threads (hereinafter referred to as a "pipe taper tap") is a kind of hand taps and is used for cutting tapered female pipe threads for achieving an air tightness of the threads at the connecting portion of pipes, pipe components or fluid apparatuses. The pipe taper tap is different from a common tapping tool for forming triangle threads, for instance, in some respects. The main differences between these two are as follows. First, the pipe taper tap cuts the pipe threads by the thread portion of the tap tapered in a ratio of 1/16 in order to taper the pipe threads in a ratio of 1/16, while the common tapping tool cuts the threads only by the chamfer. Second, the pipe taper tap cuts the threads not only by the crest but also by the root of the thread in order to improve the air tightness of the pipe thread, while the common tapping tool does not cut the threads by the root of the thread.
As described above, since the cutting area of the pipe taper tap is larger than that of a common tapping tool, the cutting resistance of the pipe taper tap is larger than that of the common tapping tool. Therefore, it is impossible to cut threads in a pipe made of a hard material, such as the one after a heat treatment having hardness of 50 HRC or more, by using a pipe taper tap.
However, sometimes a heat treatment is mistakenly conducted on the material to be tapped before cutting female threads therein. Since such a material becomes hard after the heat treatment, the body of the pipe taper tap is crimped, broken, chipped or abraded in an early stage of use by merely cutting threads in one or two holes by the pipe taper tap. In such an event, the material after the heat treatment has to be abandoned or otherwise conducted an electrical discharge machining process on the material before cutting female threads therein. However, it is difficult to accurately cut threads after the electrical discharge machining. In addition, it takes more than several ten hours to conduct the overall process.